In a general agricultural tractor, power generated by an engine is divided into traveling power for driving wheels through a transmission, and PTO (Power Take Off) power for driving various kinds of working machines mounted on the tractor.
Here, the traveling power is mainly concentrated on rear wheel drive in view of working features of the tractor. However, if greater force is required, connection is made to a front wheel drive shaft, thereby switching into four wheel drive.
FIG. 1 is a sectional view wholly showing a conventional four wheel drive switching device of a tractor. As shown in FIG. 1, the conventional four wheel drive switching device comprises a rear wheel drive shaft 10 rotated while receiving power from an engine, a first drive gear 11 for transmitting the rotational power of the rear wheel drive shaft 10, a second drive gear 20 rotated while receiving power from the first drive gear 11, a sliding gear 40 rotated while receiving power from the second drive gear 20, a front wheel drive shaft 30 coupled with the sliding gear 40 to move in a right and left direction and having a tooth guide 31 to be engaged with a toothed protrusion 45 of the sliding gear 40 so as to receive power therefrom, and a manipulation link 50 for moving the sliding gear 40 in the right and left direction to achieve four wheel drive switching.
FIGS. 2a and 2b are detailed views of a portion designated by “A” in FIG. 1. As shown in FIGS. 2a and 2b, the second drive gear 20 is formed with a large-diameter gear portion 21 that is engaged with the first drive gear 11 for power transmission, and a small-diameter gear portion 23 that is formed at a side opposite to the large-diameter gear portion 21 and is engaged with the sliding gear 40.
The sliding gear 40 comprises a large-diameter gear portion 41 that is engaged with the small-diameter gear portion 23 of the second drive gear 20; a rim groove 43 formed on one side of the large-diameter gear portion 41 so as to be connected with the manipulation link 50; and the toothed protrusion 45 formed on an inner surface of the large-diameter gear portion 41 coupled with the front wheel drive shaft 30.
FIG. 3 is a schematic view showing a conventional four wheel drive shift lever. As shown in FIG. 3, an operator manipulates a lever 51 for four wheel drive switching, and the lever 51 connected to the complicated manipulation link 50 linearly moves the sliding gear 40 toward the front wheel drive shaft 30.
However, the prior art configured as above requires space for installation and operation of the manipulation link 50 and manipulation lever 51 for operating the sliding gear 40. Thus, the size of the body of the tractor increases upon design thereof, resulting in difficulty in achieving compact design.
In addition, the number of processes of assembling the manipulation link 50 and the manipulation lever 51 increases, assembling work becomes more difficult, and time required for the assembling processes also increases, thereby causing a problem of lowered productivity.
Moreover, since the prior art has a structure in which the sliding gear 40 is connected in a stationary state to the second drive gear 20, there is a problem in that the four wheel drive switching should be made only when the tractor is stopped and the second drive gear 20 is not rotated.
In addition, since both the small-diameter gear portion 23 of the second drive gear 20 and the large-diameter gear portion 41 of the sliding gear 40 are manufactured in the form of spur gears, there are problems in that initial engagement of these gears is not easy, frictional resistance between gear teeth is significant during the engagement of the teeth, and excessive force is exerted when a user operates the manipulation lever 51.
FIGS. 4a and 4b are schematic views showing a conventional four wheel drive switching device using a hydraulic cylinder, which solves some of the problems of the prior art shown in FIG. 3. In this four wheel drive switching device, four wheel drive switching can be achieved by means of button operation.
In the conventional four wheel drive switching device using a hydraulic cylinder as shown in these figures, the sliding gear 40 that is linearly moved in the right and left direction to transmit power of the rear wheel drive shaft 10 to the font wheel drive shaft 30 is operated by a hydraulic cylinder 60 and a link 63 connected to an arm 61 of the hydraulic cylinder 60.
As for the operation of this device, although not shown in the figures, when an operator manipulates a switch to input an ON signal, a modulator valve (or a valve for regulating the flow of a working fluid) is operated in response to the signal, so that the working fluid can be supplied to the hydraulic cylinder 60 installed to a four wheel drive shift unit, and the arm 61 of the hydraulic cylinder 60 protrudes as shown in FIG. 4b to pivot the hinged link 63 through a certain angle.
At this time, the sliding gear 40 interlocked with the link 63 is linearly moved in the right and left direction, thereby achieving the four wheel drive switching.
However, this prior art has a problem of increased production costs since an expensive hydraulic cylinder 60 is additionally used, and also has difficulty in performing assembling processes due to the connection structure of the link 63.